Processing and refining various hydrocarbons streams often involves hydrotreating the streams.
For example, petroleum refiners are keen to upgrade low value coker kerosene to high value feedstock like normal paraffins. Since coker kerosene contains high levels of sulfur and nitrogen, the coker kerosene has to be hydrotreated to reduce the levels of sulfur and nitrogen before the normal paraffins can be separated from the non-normal hydrocarbons in a separation zone, such as a Molex separation unit from UOP, LLC. (Des Plaines, Ill.).
Feed stream specifications for such separation zones require severe hydrotreating to reduce the sulfur to less than 1 wppm and nitrogen to 0.5 wppm (maximum). Additionally, one of the feed stream specifications is that the Bromine Index of the feed should be in the range of 50-100. Coker kerosene also contains olefins, diolefins, and aromatics. Typical aromatics content of coker kerosene is between 20 to 30 wt %. The olefin content is also quite high and is normally designated by the Bromine Number (which is roughly about 1000 times the Bromine Index), which is typically between 50 to 55. In order to meet all three feed specifications for sulfur, nitrogen and Bromine Index, hydrotreating at pressures of 3.4 to 7.6 MPag (500 to 1100 psig) is required.
As will be appreciated, these high pressures consume significant amounts of energy, and include high operating costs. It is always preferable to perform hydrotreating at the lowest possible pressure to reduce the capital cost of the hydrotreating unit so that a refiner will receive a better return on the investment in the hydrotreating unit. While it is possible to meet the sulfur and nitrogen specifications at a relatively lower pressure, for example, 4.8 to 6.2 MPag (700 to 900 psig), such lower pressures fail to achieve the desired Bromine Index for the product.
Normally a post-treatment reactor, loaded with a hydrotreating catalyst, is required to be installed downstream of the main hydrotreating reactor to achieve the desired Bromine Index specification. The post-treatment reactor has to operate at sufficiently high pressure and catalyst volume to meet the Bromine Index. Due to equilibrium limitations, the temperature of the post-treatment reactor should be in the range of 260 to 304° C. (500 to 580° F.) to ensure the required olefins saturation is obtained to meet the required Bromine Index requirements.
It would be desirable to have one or more processes that efficiently and effectively provide effluent streams that achieve the desired Bromine Index.
Additionally, refiners are constantly seeking to reduce the capital costs and operating expenses costs for various units, such as diesel hydrotreating units. Thus, it would also be desirable to have a process which provides quality product that meets all the specifications at low energy consumption is the requirement that refiners are keen to achieve. Finally, refiners are also seeking process for producing ultra-low sulfur diesel (ULSD) at a lower cost. In conventional process, a single stage operation is used with high catalyst volume to attain a product sulfur content of less than 10 wppm. Quite often when the reactor dimensions are restricted by site considerations (maximum weight or height) more than one reactor may be required in series to meet the product specifications.
Therefore, it would be desirable to provide one or more processes which can efficiently and effectively produce ultra-low sulfur diesel.